The invention relates to a rear-projection system including a screen, at least one image source with an associated projection-lens system, and at least one mirror intended for folding the light path.
In a rear-projection system the primary image produced by an image source is projected onto a first side, referred to as the rear, of the projection screen. This image can be observed by a viewer situated at the second side, referred to as the front, of the screen.
A rear-projection screen influences both the average direction of the emerging light and the dispersal of the light in the viewing space. These functions may be performed by separate optical elements but alternatively both functions may be performed by the same element. As used herein the term "projection plate" refers to that element of the projection screen in which the light is dispersed, regardless of whether this element also influences the average direction of the emerging light.
Rear-projection systems having one image source in the form of a cathode-ray tube for monochrome reproduction or three such image sources for color reproduction are employed for displaying a video program with substantially larger picture size than attainable by means of a conventional television picture tube.
A disadvantage of rear-projection systems is the large volume of the housing which accommodates the projection screen, the image source(s) and the projection lens system(s). Depending to some extent on the dimensions of the image source and the number and the arrangement of the mirrors the volume of this housing in the case of a screen diagonal of 140 cm (45 inches) is comparable in height to that of a tall book case or china cabinet.
U.S. Pat. No. 4,491,872 describes a rear-projection system whose housing comprises an elevatable housing section. The image source, the electronic circuitry and the power supply are accommodated in a stationary lower housing section of the apparatus. The rear-projection screen and one of the mirror are accommodated in the elevatable housing section. When the apparatus is not in use the elevatable housing section having the projection screen and the mirror is collapsed into the lower stationary housing section. The apparatus then occupies only a comparatively small volume. During use the elevatable section is in the elevated position to display a picture or scene on the screen.
Although in the collapsed position this system has a far more compact appearance than a similar apparatus in which the screen occupies a fixed position, the space occupied by the elevatable type of apparatus is not reduced significantly. The floor surface occupied in both cases is the same. The space above the apparatus in the collapsed condition can be utilized for other purposes only to a very limited extent, because it should remain free to elevate the projection screen and the mirror.
A second disadvantage is the construction of the elevatable-type of apparatus. It requires a large number of additional parts such as a drive motor, a mechanical transmission etc., which renders the apparatus more expensive and heavier.